Enable -Wall option. Fix some warning.
1)fix some implicit declaration warning.
2)Rearrange serial io relative function and varible.
Remove unused function setup_hardware().
Fix implicit declaration warning. Move pvPortRealloc from memory_op.c to util.c
memory_op.c only has one function. To make it more clean and more reasonable,
this function move to util is the better choice.(According to man pages,
realloc is in <stdlib.h>)